The present invention relates to magnetic data storage and retrieval systems. More particularly, the present invention relates to a system for biasing and stabilizing a magnetic sensor with in-stack biasing structures.
In an electronic data storage and retrieval system, a magnetic recording head typically includes a reader portion having a sensor for retrieving magnetically encoded information stored on a magnetic disc. Magnetic flux from the surface of the disc causes rotation of the magnetization vector of a sensing layer or layers of the sensor, which in turn causes a change in the electrical properties of the sensor. The sensing layers are often called free layers, since the magnetization vectors of the sensing layers are free to rotate in response to external magnetic flux. The change in the electrical properties of the sensor may be detected by passing a current through the sensor and measuring a voltage across the sensor. Depending on the geometry of the device, the sense current may be passed in the plane (CIP) of the layers of the device or perpendicular to the plane (CPP) of the layers of the device. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary to recover information encoded on the disc.
An essential structure in contemporary read heads is a thin film multilayer structure containing ferromagnetic material that exhibits some type of magnetoresistance (MR). Examples of magnetoresistive phenomena include anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunneling magnetoresistance (TMR).
AMR sensors generally have a single MR layer formed of a ferromagnetic material. The resistance of the MR layer varies as a function of cos2α, where α is the angle formed between the magnetization vector of the MR layer and the direction of the sense current flowing in the MR layer.
GMR sensors have a series of alternating magnetic and nonmagnetic layers. The resistance of GMR sensors varies as a function of the spin-dependent transmission of the conduction electrons between the magnetic layers separated by the nonmagnetic layer and the spin-dependent scattering that takes place at the interface of the magnetic and nonmagnetic layers and within the magnetic layers. The resistance of a GMR sensor depends on the relative orientations of the magnetization in consecutive magnetic layers, and varies as the cosine of the angle between the magnetization vectors of consecutive magnetic layers.
A typical GMR read sensor configuration is the GMR spin valve, in which the GMR read sensor is a multi-layered structure formed of a nonmagnetic spacer layer positioned between a synthetic antiferromagnet (SAF) and a ferromagnetic free layer, or between two ferromagnetic free layers. In the former structure, the magnetization of the SAF is fixed, typically normal to an air bearing surface of the GMR read sensor, while the magnetization of the free layer rotates freely in response to an external magnetic field. The SAF includes a reference layer and a pinned layer that are magnetically coupled by a coupling layer such that the magnetization direction of the reference layer is opposite to the magnetization of the pinned layer. In the latter structure, the magnetizations of the two free layers rotate freely in response to an external magnetic field. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the reference layer of the SAF, or as a function of an angle formed between the magnetization directions of the two free layers.
TMR sensors have a configuration similar to GMR sensors, except that the magnetic layers of the sensor are separated by an insulating film thin enough to allow electron tunneling between the magnetic layers. The tunneling probability of an electron incident on the barrier from one magnetic layer depends on the character of the electron wave function and the spin of the electron relative to the magnetization direction in the other magnetic layer. As a consequence, the resistance of the TMR sensor depends on the relative orientations of the magnetization of the magnetic layers, exhibiting a minimum for a configuration in which the magnetizations of the magnetic layers are parallel and a maximum for a configuration in which the magnetizations of the magnetic layers are anti-parallel.
For all types of MR sensors, magnetization rotation occurs in response to magnetic flux from the disc. As the recording density of magnetic discs continues to increase, the width of the tracks on the disc must decrease, which necessitates a smaller sensor. As sensors become smaller in size, particularly for sensors with dimensions less than about 0.1 micrometers (μm), more of the signal is derived from magnetization rotation that occurs at the edges of the sensor rather than at the center of the sensor. This leads to several undesirable consequences. For example, conventional readers suffer from poor scaling in that as the cross-track width of the sensor decreases, the reader signal amplitude drops disproportionately faster than the decreasing width. In addition, the cross-track resolution of the sensor is deleteriously affected because the electrical width of the sensor is increased from the increased signal at the edges of the sensor. Furthermore, manufacturing defects are more likely to be present near the edges of the sensor, which may result in a noisy reader signal. Magnetic sensors must be designed in such a manner that even small sensors are free from magnetic noise and provide a signal with adequate amplitude and accurate recovery of data written on the disc.